Forgery-proof marking for objects and method for identifying such a marking

ABSTRACT

The invention relates to forgery-proof marking for objects, such as check cards, banknotes, labels, and the like, comprising a plastic transparent film ( 1 ) having a first and second surface, whereby a series of layers is applied to the second surface. When viewed from the first surface, the color of this series of layers changes according to the viewing angle, and the series of layers is formed from an absorber layer provided on the second surface, from a spacer layer ( 3 ) overlying the absorber layer, and from a mirror layer ( 2 ) overlying the spacer layer ( 3 ). In order to improve the machine identification of the authenticity of the marking, the invention provides that the absorber layer is comprised of metallic clusters ( 4 ).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Stage application under 35 U.S.C. §371and claims benefit under 35 U.S.C. §119(a) of International ApplicationNo. PCT/EP02/09124 having an International Filing Date of Aug. 14, 2002,which claims the benefit of priority of DE 102 08 036.4 having a filingdate of Feb. 26, 2002, DE 102 05 152.6 having a filed date of Feb. 7,2002, and International Application No. PCT/DE01/03205 having anInternational Filing Date of Aug. 16, 2001.

The invention relates to a forgery-proof marking according to thepreamble of claim 1. It also relates to a method for the machineidentification of such a marking.

A similar marking is disclosed by WO 01/53113 A1. This essentiallycomprises the combination of a holographically structured film with aseries of layers which appears in different colors, depending on theviewing angle. The series of layers comprises an absorber layer, adielectric layer and a reflective layer. The layer thickness of theaforementioned layers is in each case in the nanometer range. Theproposed forgery-proof marking is intended to be used in particular forthe identification of banknotes, check cards and the like. In the caseof such a use, it is required that the authenticity of the marking beverifiable securely and reliably by machine. The aforementioned markingdoes not meet this requirement.

It is an object of the invention to specify a forgery-proof marking anda method for identifying such a marking with which the disadvantagesaccording to the prior art are eliminated. The intention is, inparticular, to specify a forgery-proof marking whose authenticity can beverified securely and reliably by machine. A further aim of theinvention is the specification of a method for identifying such aforgery-proof marking.

According to the invention, provision is made for the absorber layer tocomprise metallic clusters. This advantageously achieves the situationwhere the authenticity of the marking can be verified by machine. Theabsorber layer formed of metallic clusters produces a highlycharacteristic absorption spectrum because of the unexpected variationsof the refractive index and of the extinction against the wavelength.For example, specific peaks and/or peak shifts and/or peak forms causedby the metallic clusters can be measured. Furthermore, the metallicclusters, because of their extreme extinction coefficients, produceparticularly high intensities of the peaks in the absorption spectrum ascompared with the conventional unstructured absorber layers. In aconventional unstructured absorber layer, the absorption is onlyslightly dependent on the angle over wide angle ranges, as is known.When the structured absorber layer comprising metallic clustersaccording to the invention is used, it is found that this intrinsicallyexhibits a substantially more intense angle-dependent absorption. As aresult, the absorption spectrum of the forgery-proof marking accordingto the invention changes in a manner which is unexpected and can beverified by machine when measured at different angles. Theaforementioned properties of the forgery-proof marking permit secure andreliable machine verification of the authenticity.

It has proven to be expedient for the clusters to form discrete islandswith a size of at most 100 nm, preferably 5 to 35 nm, in at least onespatial direction. The thickness of the, preferably dielectric, spacerlayer is expediently chosen such that the absorption of visible lightincident on the cluster layer is a maximum.

According to a further configuration feature, it has proven to beexpedient for the series of layers to have, at a viewing angle of 45° inthe wavelength range between 300 and 800 nm, an absorption with amaximum value of at least 60%, preferably 80%, particularly preferably90%. This permits secure and reliable machine identification of theforgery-proof marking.

The clusters are expediently formed from one of the following metals:gold, silver, platinum, palladium, tin, aluminum, copper, indium.

According to a further refinement, the cluster layer can not only bejoined firmly but also joined detachably to the spacer layer. It is alsopossible for the spacer layer not only to be joined firmly but alsojoined detachably to the mirror layer. The proposed configurationspermit reversible separation of the series of layers. In the conversecase, however, it is also possible to join the series of layersreversibly at the proposed separation surfaces. This makes it possiblefor the forgery-proof marking to be visible only when read out.

With regard to the spacer layer, it has proven to be expedient for thisto have a thickness of 40 to 2000 nm. The spacer layer can be producedfrom one of the following materials: metal oxide, metal nitrite, metaloxynitrite, metal carbide, in particular from silicon oxide, siliconcarbide, silicon nitrite, tin oxide, tin nitrite, aluminum oxide,aluminum nitrite or polymer, in particular polycarbonate (PC),polyethylene (PE), polypropylene (PP), polyurethane (PU), polyimide(PI), polystyrene (PS) or polymethacrylate (PMA), polyvinyl alcohol(PVA), polyacrylates (PA), nitrocellulose (NC), polyethyleneterephthalate (PET).

According to a further refinement, the film has a layer thickness of 5to 100 μm. It can be produced from polyethylene terephthalate. Accordingto a particularly advantageous refinement, the first or the secondsurface of the film has a structure in order to produce a holographiceffect. The structure size for producing a holographic effect can be inthe range from 0.1 to 1.0 μm. With the proposed implementation, aholographically structured film can therefore be provided with aforgery-proof color marking whose authenticity can be identified bymachine.

According to a further configuration feature, the mirror layer can beapplied to a carrier film, it being in turn possible for an adhesivelayer to be applied to the carrier film, for example by means of alamination operation. This makes it possible to stick a forgery-proofmarking formed in such a way onto an object to be marked.

The adhesive layer is expediently produced from a pressure-sensitiveadhesive or a hot-melt adhesive. The adhesive layer is advantageouslycovered with a protective film that can be pulled off. This makes iteasy to affix the forgery-proof marking to an object to be marked.

According to a further refinement, it is also possible for the series oflayers applied to the second surface to be present in the form oflayered flakes which are accommodated in a transparent matrix. Thelayered flakes can, for example, float in a regular arrangement in atransparent plastic matrix. They can be applied to the film dispersed ina clear plastic varnish.

The series of layers, for example beginning with the cluster layer, isadvantageously applied directly to the surface of the film by means of acoating operation. In such coating operations, it proves to beadvantageous if the films can be wound up and guided continuously orsemi-continuously through a coating installation. Whereas in the case ofdiscontinuous coating, for example directly on products, high unitprices must be expected, coating processes, such as vacuum coatingprocesses, can be used relatively cost-effectively in the continuousmode. In this case, reflective films are particularly suitable since, ifthey are used, at least some of the mirror action necessary forproducing the characteristic color effect is achieved by the surface tobe marked.

If the distance between the film and the cluster layer is less than 2μm, a coloration forming the marking becomes visible. The thickness ofthe spacer layer is therefore preferably between 20 and 2000 nm. In thethree methods described, it is expediently applied in structured form.The structuring can be a structure in the surface in the manner of aline of text, a pattern, for example a bar code, or a drawing, forexample a logo. However, it can also be a relief-like structure. In thiscase, the marking appears in different colors. The application of thin,preferably polymeric layers with non-vacuum-based methods permits simpleproduction of such a relief-like structured spacer layer.

According to a further configuration feature, an inert protective layerthat is transparent to electromagnetic waves is applied to the clusterlayer. The protective layer is used for protection against mechanicaldamage and contamination. However, it also influences the characteristiccolor spectrum in a defined way and, as a result, increases thecomplexity of the layer structure and therefore the security againstforgery of the marking.

The protective layer can be produced from one of the following materialtransparent to electromagnetic waves: polymer, metal oxide, metalnitride, metal oxynitride, metal carbide, metal fluoride, in particularfrom silicon oxide, silicon carbide, silicon nitride, tin oxide, tinnitride, aluminum oxide or aluminum nitride. These materials aresubstantially chemically inert and insensitive to moisture.

According to a further configuration feature, provision is made for thelayers of the series of layers to be produced, at least to some extent,by means of thin layer technology. In this case, in particular PVD, CVDmethods and the like are suitable. In addition, it is also possible todeposit the layers of the series of layers from solutions by a wetchemical route. To this extent, reference is made to WO 98/48275, whosedisclosure content is hereby incorporated.

According to a further refinement, provision is made to process a filmcoated with the series of layers to form adhesive or laminating labels.To this end, the film is provided on one of its two sides with anadhesive layer or a double-sided adhesive film or a laminating layer.According to an exemplary application, the layer system produced in thisway is then applied to a siliconized substrate with the adhesive layerat the bottom. After that, any desired shapes can be punched or cut fromthe layer system without affecting the stability of the siliconizedsubstrate. The excess parts can then be removed by stripping, as aresult of which the layer system in self-adhesive form or in a formwhich can be laminated can be applied to different products in anautomated form over dispensing edges.

The forgery-proof marking can be used in particular for films forprocessing in check cards, banknotes, labels for valuable products, forexample, or their packages and the like. In this case, a spacer layerwith a predefined thickness is applied to a mirror layer joined to thefilm. Furthermore, a metallic cluster layer is applied to the spacerlayer. Such a marking is permanently visible; it is very forgery-proof.

The forgery-proof marking can also have, on a cluster layer joined tothe film, a spacer layer applied thereto with a predefined thickness anda mirror layer lying above that. Such a marking is permanently visiblethrough the film; it too is very forgery-proof.

The forgery-proof marking can also have a mirror layer joined to thefilm and a spacer layer with a predefined thickness. Such a marking isinitially invisible. A cluster layer can be applied to a further film assubstrate in such a way that, for the purpose of verification or inorder to make the marking visible, it can be arranged at a predefineddistance from the first layer.

The film to be marked is, for example, produced from a plastic such aspolycarbonate, polyurethane, polyethylene, polypropylene, polyacrylate,polyimide, polyvinyl chloride, polyepoxide, polyethylene terephthalate,or from a metal such as aluminum, gold, silver, copper, iron orstainless steel.

If the film to be marked is already produced from a material thatreflects electromagnetic waves, for example a metal, or is coated withsuch a material, the mirror layer can be formed by the film itself.

The film to be marked can be printed before the coating, it beingpossible for the optical effects of the marking layer system to beinfluenced in an unexpected way by the interaction with the printingink. In accordance with the invention, it proves to be an advantageousrefinement if the layer that reflects electromagnetic waves and also thecluster layer exhibit less than 50% reflectance over at least some ofthe visible spectrum.

In general, the use of printing methods can serve to store additionalinformation on the marking surface.

As a result, personalized marking can also be achieved. Suchpersonalization of the marking can also be achieved subsequently byprinting the marked surfaces with printing methods that are morewidespread such as laser and inkjet printers.

The films to be marked can also be provided with holograms. The markingcan advantageously be formed in such a way that all the marking layerstogether absorb less than 90% of the incident light and thus thehologram structures lying underneath can still easily be detected.Furthermore, the marking described can also be provided directly on orin the vicinity of the embossed surface of holograms, as a result ofwhich the holograms become forgery-proof and machine-readable.

According to the invention, a method is also provided for the machineidentification of a forgery-proof marking according to the invention,having the following steps:

-   a) registration of the spectrum of the light reflected by at the    forgery-proof marking at a predefined viewing angle,-   b) measurement of values for determining (i) the position    and/or (ii) the shape and/or (iii) the intensity of one or more    absorption peaks characteristic of the marking within a predefined    spectral range and-   c) comparison of the values (i) to (iii) measured in step b with    predefined corresponding values and-   d) identification of the marking by using the result of the    comparison.

The measurement of the values for determining the characteristicabsorption peaks is carried out in accordance with step b within apredefined spectral range. Here, the spectral range in which theabsorption peaks characteristic of the marking are expected isexpediently chosen. If no absorption peak occurs in this predefinedspectral range, it is possible to omit the further steps foridentification. Steps c and d contribute to high identificationsecurity.

It has proven to be expedient to register the spectrum at a viewingangle of 5 to 50°, based on the normal to the surface, preferably of 15°to 40°. Particularly pronounced absorption peaks are observed in thisrange. Furthermore, it is expedient to use the symmetry of theabsorption peak as a detection feature for the presence of an absorptionpeak produced by the cluster layer. Absorption spectra which areproduced by cluster layers are partially formed distinctlyasymmetrically.

As a further identification feature, the absolute intensity of theabsorption peak is measured. This is particularly high as compared withabsorption peaks of series of layers produced in accordance with theprior art.

The light shone onto the marking can be produced by means of anincandescent lamp, laser, fluorescent lamp, light-emitting diode orxenon lamp. In this case, the reflected light is particularly wellsuited to measuring the absorption spectra. In order to increase thesecurity of the identification, the marking can be identified byregistering the reflected spectrum at various viewing angles.

In order to further secure the authenticity of the marking, the markingcan be identified as such only if the measured values (i) to (iii) liewithin a predefined value range around the corresponding values.

Owing to the further configuration features of the method, reference ismade to the preceding explanations relating to the forgery-proofmarking.

In the following text, exemplary embodiments of the invention will beexplained in more detail using the drawings, in which:

FIG. 1 shows a schematic cross-sectional view of a first continuouslyvisible marking,

FIG. 2 shows a schematic cross-sectional view of a first marking that isnot continuously visible and also a second film suitable forverification or affording visibility,

FIG. 3 shows a schematic cross-sectional view of a continuously visiblefirst marking that can be laminated or bonded adhesively,

FIG. 4 shows a schematic cross-sectional view of a further continuouslyvisible second marking that can be laminated or bonded adhesively,

FIG. 5 shows a schematic cross-sectional view of a first marking that isnot continuously visible and can be laminated or bonded adhesively, andalso a second film suitable for verification or affording visibility,

FIG. 6 a shows a strip coating with cluster marking,

FIG. 6 b shows adhesive labels produced from strip as in FIG. 6 a,

FIG. 7 shows absorption spectra of a forgery-proof marking at variousviewing angles,

FIG. 8 shows a quantitative evaluation of the spectra according to FIG.7 at various wavelengths,

FIG. 9 shows measured absorption spectra of forgery-proof marking withmetallic cluster layers of different thickness, and

FIG. 10 shows calculated absorption spectra of markings with metallayers of different thicknesses.

FIG. 11 a shows five forgery-proof marking applied to an aluminumsubstrate, which cannot be identified unambiguously by eye.

FIG. 11 b shows measured absorption spectra of the five forgery-proofmarkings from FIG. 11 a.

In the markings shown in FIGS. 1 to 5, a mirror layer reflectingelectromagnetic waves is designated by the reference symbol 2. This canbe a thin layer of aluminum, for example. The mirror layer 2 can,however, also be a layer formed of metallic clusters, which is appliedto a film 1. The film 1 can be the film to be marked. An inert spacerlayer is designated 3. The cluster layer is designated by the referencesymbol 4.

In FIGS. 2 and 5, the second film for making the marking visible isprovided with the reference symbol 5.

In FIGS. 3 to 5, the adhesive or laminating layer provided for thefurther processing of the forgery-proof marked film is designated 6. Thechange in the reflected light, producing the characteristic colorspectrum, as compared with the incident light is visualized in FIGS. 3 &4 by means of the gray-stepped variation in an arrow.

In the markings shown in FIGS. 1 and 3, the cluster layer 4 is appliedto the spacer layer 3. In this case, the spacer layer 3 is applied tothe mirror layer 2. Furthermore, in FIGS. 1 and 3, the mirror layer isapplied to a film 1.

In FIG. 4, first of all the cluster layer 4, then the spacer layer 3,then the mirror layer 2 and lastly the adhesive or laminating layer 6are applied to the film 1.

In the markings shown in FIGS. 2 and 5, only the optically transparentspacer layer 3 is applied to the mirror layer 2 and the latter isapplied to a film 1. The marking is initially not visible. The markingsare only visible when they are brought into contact with the second film5, to whose surface the cluster layer 4 formed of metallic clusters isapplied. A color effect, which can be observed through the transparentfilm 5, is then in turn produced. The film 5 is expediently producedfrom a transparent material, for example from plastic such aspolycarbonate, polyurethane, polyethylene, polypropylene, polyacrylate,polyvinyl chloride, polyepoxide, polyterephthalate.

The function of the marking is as follows:

When light from a light source, such as an incandescent lamp, laser, afluorescent tube or a xenon lamp, is shone onto one of the markingsshown in FIGS. 1, 3 and 4, this light is reflected at the mirror layer2. As a result of interaction between the reflected light and thecluster layer 4 formed of metallic clusters, some of the incident lightis absorbed. The reflected light has a characteristic spectrum thatdepends on a number of parameters, such as the optical constants of thelayer structure or the shape of the clusters. The marking appearscolored. The coloration serves as a forgery-proof verification of theauthenticity of the marking. The color impression obtained in this waydepends on the angle and can be identified both roughly with the nakedeye and exactly with a reading device operating in the reflection mode,preferably a spectrophotometer. Such a reading device can, for example,register the coloration of the marking from two different angles. Thisis done either by means of a detector by two light sources which areconnected up appropriately being used and the detector being tiltedappropriately, or by two reading devices measuring the sample,illuminated from two different angles, from the two correspondingangles.

With regard to the parameters to be maintained for the production of theinteractions, reference is made to U.S. Pat. No. 5,611,998 and WO99/47702, whose disclosure content is hereby incorporated.

FIG. 6 a shows a continuously coated forgery-proof marked film which haspartly been wound up on rolls.

FIG. 6 b illustrates how adhesive labels produced from a film as in FIG.6 a were fabricated with the forgery-proof marking.

The spectra shown in FIG. 7 from a forgery-proof marking according toFIG. 1 were measured by means of a Lambda UV/VIS spectrometer fromPerkin Elmer, using a reflection insert. FIG. 7 reveals that thelonger-wave peak is displaced toward shorter wavelengths as the viewingangle increases. Furthermore, a stationary peak can be observed, whichis characteristic of the silver clusters used. At viewing angles of lessthan 45°, the peaks of this marking have an intensity of about 1 OD,which corresponds to 90% absorption.

FIG. 8 shows a quantitative evaluation of the spectra according to FIG.7, in each case at two different wavelengths. At the wavelengthsconsidered, a changed absorption is observed as a function of theviewing angle. The absorption pattern is characteristic of theauthenticity of the marking.

FIGS. 9 and 10 again illustrate the difference of the cluster layersaccording to the invention as compared with conventional absorber layerswhich are formed from a metal layer. The spectra shown in FIG. 9 havebeen measured on a forgery-proof marking which has a film produced frompolyethylene terephthalate with a thickness of 75 μm. A gold layer witha thickness of 100 nm is applied to this film as a mirror layer. Themirror layer is covered with a spacer layer produced from MgF₂ with athickness of 270 nm. The spacer layer is in turn covered by a layerproduced from metallic gold clusters with thicknesses in the range from0 to 12 nm. The aforementioned layers were applied to the film by meansof vacuum coating. The measurements were in each case carried out at aviewing angle of 18°.

By comparison, FIG. 10 shows absorption spectra which have beencalculated by using the aforementioned parameters for an absorber layerproduced from gold.

A comparison of FIGS. 9 and 10 shows that, in this case, in particularcluster layers with a thickness in the range from 2.5 to 5 nm exhibit acharacteristic absorption peak displaced toward higher wavelengths. Theabsorption peak is broadened considerably and, in the case of the 5 nmthick cluster layer, is asymmetrical. At 8 nm cluster thickness, theabsorption peak is at the same wavelength as in the calculated spectrumbut still considerably higher. At higher thicknesses of the clusterlayers, the absorption peaks are similar to the absorption peaks of thecalculated spectra. This points to the fact that, for the caseillustrated here, starting at a thickness of about 12 nm, the coatingdensity of the clusters is so high that the cluster layers formedbehave, at least optically, like continuous metal layers.

The forgery-proof marking proposed by the invention can be identified bymachine with high reliability. For this purpose, the marking isirradiated, for example by means of an incandescent lamp. The absorptionspectrum of the light reflected from the marking is measured at aviewing angle of, for example, 18°. For this purpose, a spectral rangebetween 500 and 700 nm is advantageously observed. The absoluteintensity of an absorption peak possibly occurring there is determined.Furthermore, the spectral position of the maximum is determined. Inaddition, the symmetry of the absorption peak can be determined by usingpredefined reference points. The values determined are compared withpredefined value ranges which have been determined by using standards,for the purpose of identification of the marking.

In order to increase the identification security, the aforementionedmeasurement can be carried out at different viewing angles.

FIG. 11 a shows a five-stripe sample (gold clusters on aluminum oxidespacer layer on aluminum mirror) to demonstrate the resolution ofmachine evaluation. At about 45°, all five stripes appear blueish. Thedifference between the stripes is itself barely or not visible to theeye in the gray-step mode.

FIG. 11 b shows the measured spectra of the five stripes from FIG. 11 a,which were measured with a hand-held two-channel spectrometer. Stripes1, 2, 4 and 5 are detected as forgeries during the software-assistedevaluation of the data from the two-channel spectrometer if the datafrom stripe 3 is stored as original.

LIST OF REFERENCE SYMBOLS

-   1 Film-   2 Mirror layer-   3 Spacer layer-   4 Cluster layer-   5 Second film-   6 Adhesive layer

1. A method for the machine identification of a forgery-proof markingfor an object, said forgery-proof marking comprising (a) a transparentfilm produced from plastic and having a first and second surface, (b)wherein said second surface comprises a series of layers whose colorchanges as a function of the viewing angle, wherein the series of layersis formed from an absorber layer, a spacer layer overlying the absorberlayer and a mirror layer overlying the spacer layer, wherein theabsorber layer comprises metallic clusters, said method having thefollowing steps: a) registering a spectrum of light reflected by theforgery-proof marking at a predefined viewing angle, b) measuring valuesfor determining (i) a position and/or (ii) a shape and/or (iii) anintensity of one or more absorption peaks characteristic of the markingwithin a predefined spectral range, c) comparing the values (i) to (iii)measured in step b) with predefined corresponding values, and d)identifying the marking by using results of the comparison.
 2. Themethod as claimed in claim 1, the spectrum being registered at a viewingangle of 5° to 50°.
 3. The method as claimed in claim 2, the spectrumbeing registered at a viewing angle of 15° to 40°.
 4. The method asclaimed in claim 1, the absorption peak being used as a detectionfeature for the presence of an absorption spectrum produced by thecluster layer (4).
 5. The method as claimed in claim 1, the absoluteintensity of the absorption peaks being measured.
 6. The method asclaimed in claim 1, the marking being identified as such only if themeasured values (i) to (iii) lie within a predefined value range aroundthe corresponding values.
 7. The method as claimed in claim 1, the lightbeing produced by means of an incandescent lamp, laser, fluorescentlamp, light-emitting diode or xenon lamp.
 8. The method as claimed inclaim 1, the marking being identified by registering the reflectedspectrum at various viewing angles.